Conventional printed circuit boards of the FR 4-type require comparatively large layer thicknesses for good strength values. In conjunction with the relatively poor dielectric properties, they allow only a low component density of the electronic components to be assembled.
Conventional standard printed circuit boards cannot be used for many demanding applications in the modern world of electronics, for example mobile telephones, laptops or mobile electronic game devices and in proximity radar devices in cars.
High-performance printed circuit boards based on PTFE-coated fibre glass, optionally with a PTFE foil lamination are generally used in applications of this type. In addition, high-performance polymer foils are used, which are produced based on polyethylene, liquid crystalline polymers (LCP), polyetheretherketone (PEEK), polyetherimide (PEI) or polyimide (PI). High-power printed circuit boards of this type, also in the form of flexible foils, have many improvements in comparison to the standard printed circuit boards (called FR 4-printed circuit boards in brief), but cannot, however, satisfactorily meet all requirements.
Printed circuit boards based on polymer and high-power polymer foils may be very thin and, owing to their flexibility, allow innovative installation situations, but even they still have significant drawbacks.
The PTFE coated glass fibres, with a relative dielectricity constant εr of about 2.4, do not have the optical dielectric properties of PTFE.
Air inclusions in the PTFE-coated glass fibres, so-called microvoids, may lead to defects when a voltage is applied. The production of microvoids is due to the special coating method which requires a repeated immersion of the fibre glass in a PTFE dispersion with subsequent drying and sintering. Residual contents of the emulsifiers contained in the PTFE dispersion and traces of their decomposition products remain in the PTFE-coated fibre glass printed circuit board and impair the performance potential of the material PTFE in this application. In particular, these residues influence the dielectric loss factor (tan δ) and the relative dielectricity constant εr.
PTFE-coated fibre glass has a comparatively rough surface as in the typically used application quantities the fibre glass still shows on the surface.
Rough surfaces, such as occur, in particular in the PTFE-coated glass fibres, after the application of the copper layer in conjunction with a so-called skin effect, lead to a comparatively high dielectric damping coefficient (high tan δ). Skin effect is taken to mean the fact that the (negative) charge carriers responsible for the current flow, because of their mutual repulsion, preferably flow on the surface of an electric conductor. The skin effect increases with increasing frequency. The demands on the surface quality also increase in the same direction.
Rough surfaces are also determined in PTFE foils produced by means of a peeling process. The roughness is produced here, in particular by longitudinally oriented grooves, in relation to the peeling process, originating from the peeling knife. The mechanical properties of PTFE foils produced by means of a peeling process therefore frequently differ when comparing the longitudinal and transverse values. The values determined in the transverse direction, in particular the values for tensile strength and elongation at break, may be up to 50% lower than the values determined in the longitudinal direction.
Printed circuit boards, which are produced based on polymers and high-performance polymers, are typically distinguished by good surface properties. However, the tendency of many polymers to absorb water in the course of time in direct contact with water or else air humidity has a negative effect on the properties of the printed circuit boards. The performance data of the original dry state continuously decrease during the service life because of the continuing absorption of water. The relatively pronounced tendency of the polyimide (PI) to absorb water proves to be particular negative for printed circuit board applications. In the PI-based carrier foils, a Cu foil is conventionally rolled onto the PI-carrier foil, which, for reasons of process technology, makes layer thicknesses of 18 μm and above necessary. If the provided use of the printed circuit board requires lower Cu layer thicknesses, the superfluous Cu layer thickness has to be removed again by a complex etching process in a subsequent process step.
Liquid crystalline polymers (LCP) are distinguished by a strongly reduced tendency to absorb water. In the most favourable case, in comparison with PI, a water absorption which is reduced by up to a factor of about one hundred can be observed. However, LCPs are comparatively brittle, and in particular in applications with vibration loading, mechanical damage or even a printed circuit board break may easily occur. Moreover, LCPs are very expensive materials and therefore limited to niches in printed circuit board construction.